Lesson 5: Aromatic Hydrocarbons

• Aromatic hydrocarbons were named because when first discovered, they were groups of oils with fragrant odors
• Current classification of aromatic hydrocarbons includes only compounds that have a benzene ring
• Any oil with a fragrant odor was considered an aromatic, but now all benzene ring compounds are classified as aromatics
• Benzene has 6 carbons and 3 double bonds
• Benzene's molecular formula is C6H6
• Initially, benzene was thought to have 2 double bonds or a triple bond, but neither structure was supported by its properties
• Kekule proposed that benzene is a hybrid of the 2 structures he suggested, with delocalized valence electrons
• Due to delocalized electrons, benzene is stable and unreactive
• Benzene is planar, nonpolar, and has all C-C bonds of the same length
• Rules for naming aromatics:

  • Simple alkyl groups bonded to a benzene by an end carbon are named as alkyl benzene
  • Benzene as a branch is named as a phenyl alkane
  • Distributed benzene compounds are named by numbering carbons on benzene to get the lowest possible numbers for the branches
  • Multisubstituted benzene compounds are named by determining the lowest sets of numbers for each branch
• Practice naming examples:

  • 1-methyl-3-propylbenzene
  • m-methylpropylbenzene
• o- is a prefix when substituents are attached at 1 and 2
• m- is a prefix when substituents are attached at 1 and 3
• p- is a prefix when substituents are attached at 1 and 4